Recently, chemical amplification type DUV (deep ultra violet) photoresists have proven to be useful to achieve high sensitivity in processes for preparing micro-circuits in the manufacture of semiconductors. These photoresists are prepared by blending a photoacid generator with polymer matrix macromolecules having acid labile structures.
According to the reaction mechanism of such a photoresist, the photoacid generator generates acid when it is irradiated by the light source, and the main chain or branched chain of the polymer matrix in the exposed portion is reacted with the generated acid to be decomposed or cross-linked, so that the polarity of the polymer is considerably altered. This alteration of polarity results in a solubility difference in the developing solution between the exposed area and the unexposed area. For example, in the case of a positive photoresist, the main or branched chain of the polymer matrix is decomposed by acid in the exposed area and is removed by being dissolved in the developing solution. On the other hand, in the unexposed area, the original structure of the polymer is maintained without being dissolved in the developing solution. As a result, a positive image of a mask is formed on the substrate. In the case of a negative photoresist, the main or branched chain of the polymer matrix is cross-linked, so the exposed area is not dissolved in the developing solution. As a result, a negative image of a mask is formed on the substrate.
As explained above, a chemical amplification type photoresist must comprise a substantial amount of photoacid generator. However, the use of a photoacid generator has several drawbacks: (1) photoacid generators exhibit several problems such as storage stability, gas production, occurrence of "T-topping" of the pattern owing to acid diffusion, high post-baking temperature, and the like; (2) high integration of a semiconductor element is restricted because the E/L (energy latitude) margin of the photoresist pattern must be decreased due to acid diffusion; and (3) the generated acid may combine with amine compounds to contaminate the photoresist composition, thereby deforming the pattern or altering the line width.
Furthermore, since conventional negative photoresist compositions use an organic solvent, such as xylene, as the developing solution, swelling of photoresist film (the phenomenon that developing solution is soaked into the exposed area of the photoresist) occurs during the developing process.